Does that mean that the pilot can pull an additional 1.5 MAP over the determined MAP limits when operating with full carb heat?
I believe that is the correct interpretation.
The power produced by the engine is function of how much air and fuel is in the cylinder. Make the air hotter (and less dense) you need to get more in there for the same power. So for the same actual power in the engine (and hence pressure/force which is the true mechanical limit) you need the throttle open more and that gives a higher manifold pressure.
Now ducking head under pillow .....